Implantable medical devices, such as ventricular assist devices, are being developed for long term treatment of chronic heart failure. Such devices require a pumping mechanism to move blood. Due to the nature of the application, the pumping mechanism must be highly reliable. Patient comfort is also a significant consideration.
Electrically powered pumping mechanisms typically rely on a motor such as a brushless DC motor. Brushless DC motors offer maintenance advantages in implant applications due to the lack of wear-prone brushes. Due to the lack of these electro and mechanical commutation components, commutation is generally provided electrically by drive electronics.
A prior art HeartWare Ventricular Assist System, manufactured by HeartWare Inc, Framingham Mass., is an example of an implantable ventricular assist device. At the core of the HeartWare Ventricular Assist System is a small implantable centrifugal blood pump called a HVAD® pump employing a brushless DC motor.
When implanted in a patient in a typical scenario, the pump draws blood from the left ventricle and propels that blood through an outflow graft connected to the patient's ascending aorta. The device is capable of generating up to 10 liters of blood flow per minute. With a displaced volume of only 50 cc, the HVAD pump is suitable for implantation in the pericardial space, directly adjacent to the heart. Implantation above the diaphragm leads to relatively short surgery time and quick recovery.
The HVAD pump has only one moving part, an impeller, which spins at a rate between 1800 and 4000 revolutions per minute. The impeller is suspended within the pump housing through a combination of passive magnets and hydrodynamic thrust bearings. This hydrodynamic suspension is achieved by a gentle incline on the upper surfaces of the impeller blades. When the impeller spins, blood flows across these inclined surfaces, creating a “cushion” between the impeller and the pump housing. There are no mechanical bearings or any points of contact between the impeller and the pump housing.
Device reliability is enhanced through the use of dual motor stators with independent drive circuitry, allowing a seamless transition between dual and single stator mode if required. The pump's inflow cannula is integrated with the device, and surgically implanted into the heart's ventricle. This proximity is expected to facilitate ease of implant and to help ensure optimal blood flow characteristics. The use of a wide-bladed impeller and clear flow paths through the system minimizes risk of pump-induced hemolysis (damage to blood cells) or thrombus (blood clotting).
Typically, while the pump is implanted in the patient, a controller and the drive electronics for the pump, and other control subsystems for the pump, including the power supply, are located outside the patient, for example, in a control/power supply module tethered by a transcutaneous electrical cable, to the implanted pump of the overall HeartWare Ventricular Assist System
For the HeartWare Ventricular Assist System, an external (to the patient) controller includes the drive electronics for the pump (coupled directly to the windings of the motor) and provides drive and control signals to the pump. The controller also provides feedback and alarms to the patient regarding the operation of the device. Commutation control for the brushless DC motors is effected by the controller and the drive electronics, in a feedback manner. The controller provides a commutation control signal for a selected phase of the motor in accordance with a sampled back-emf voltage of that phase (sensed via the tether cable). The back-emf is sampled only while the corresponding selected phase drive voltage is substantially zero. The frequency of the brushless DC drive voltage is varied in accordance with the commutation control signal. In one form, the back-emf is normalized with respect to a commanded rotor angular velocity. A speed control generates a speed control signal corresponding to a difference between a commanded angular velocity and an angular velocity inferred from the frequency of the drive voltage.
A redundant power supply is provided by two batteries, or one battery and an AC adapter or DC adapter. The redundant power supply provides power for the controller, and particularly the drive electronics. When the battery is depleted (for example, after approximately 6 hours), the controller automatically switches to the standby power source, battery or adapter, and the depleted battery is replaced.
A “Patient Pack” assembly includes a carrying case that holds the controller and power source(s). The case can be adapted to be carried over the patient's shoulder or worn around the patient's waist.
While the prior art HeartWare Ventricular Assist System, in the aggregate, performs the desired ventricular assist functions required for long term treatment of chronic heart failure, there is a need for improved subsystems and subassemblies which would provide enhanced blood flow results and improved patient-convenience features, easing the maintenance burden on the patient, thereby providing an improved quality of life.